The invention relates to a plasma torch with a microwave transmitter, according to the kind of patent claims which, for example, is used to coat surfaces and to produce radicals.
Known magnetron-ion sources employ a magnetron for generating an alternating electric field; refer to DE 37 38 352 A1. It is an disadvantage that a quartz dome and external magnetic fields are required to generate the gas plasma. The intensive magnetic field in the discharge chamber is used to match the cyclotron frequency to that of the microwave generator. The operation of the microwave gas discharge takes place without electrodes. Furthermore, the operation requires a cooling of the device. Such plasma generators are of a complex structure and are limited in their dimensions. The technical expenditures for microwave gas discharges systems are high. It is not feasible to transmit high powers, and it is not evident that plasmas of high density are stable when high powers are concerned.
Devices for generating plasmas by microwaves, as known from, for example, DE 3905303 C2, DE 3915477 C2. U.S. Pat. No. 5,349,154 A, generally use quartz tubes. A magnetron (microwave transmitter unit) is secured to one end of a rectangular hollow guide. The generated microwaves pass through the hollow guide and impinge, at the other end of the hollow guide, upon a quartz glass insert through which a special gas flows. The flowing originates from a low pressure maintained in the recipient. In the quartz glass insert a plasma is generated by the microwave energy, and the plasma flows through the quartz glass insert into the recipient. The method is characterized by not having any electrodes. Such devices exhibit the following disadvantages:
The hottest site and the center of the plasma are located in that portion of the quartz glass insert, which is arranged within the rectangular hollow guide. Hence, the energy is transformed before the recipient rather than within the same and, at a respective application, too little radicals are provided for the operation process.
A high rate of wall effects occur within the quartz glass.
The mass throughput and the effective pressures of 500 Pa to 3 kPa are too low.
The quartz glass insert is not suited for any large-scale technical continuous operation. Due to the unintentional high temperatures the quartz glass insert shows melting effects, or there have to be additionally provided expensive cooling devices.
The efficiency of the energy exploitation is low.
It is difficult to maintain the vacuum tightness at the sealing faces.
In the course of mounting and dismounting, respectively, and due to the thermal expansion of the metallic components it can be possible that the glass will be destroyed.
Furthermore, devices are known, in which a cross-coupling of a rectangular hollow guide with a coaxial guide is provided. Also in this case, a microwave generating device and a microwave transmitter device, respectively, i.e. a magnetron, are secured to one end of a hollow guide. The generated microwaves pass through the hollow guide and impinge upon a conductive longitudinally extending nozzle. The hollow guide is closed by a short-circuit slide. In this way, the resulting electromagnetic wave is tunable. Such a known arrangement can be designed with a quartz tube (DE 195 11 915 C2) or without one (U.S. Pat. No. 4,611,108 A). Apart from the fact that when using quartz tubes the specific disadvantages occur as mentioned above, this cross-coupling features the following disadvantages:
The exploitation of the microwave output is of low efficiency.
Energy losses occur at the cross-coupling between the rectangular hollow guide and the coaxial guide.
The entire construction is complicated.
The maximal operation pressure and the mass throughput are too little.
From U.S. Pat. No. 4,473,736 A a plasma generator is known, in which a cavity and a coaxial guide are capacitively coupled. Insulating thin disks supporting the electrode are arranged distributed along the entire cross-section of the cavity and the coaxial guide. Apart from not being a hollow wave guide, this arrangement is not suited for an impedance matching and for obtaining a low-reflective hollow wave conduction.
Hence, it is an object of the present invention to provide a plasma torch that generates plasma with high densities in a range near normal pressure. Thereby high powers are capable to be transmitted. A stable combustion and an efficient exploitation of the microwave energy shall be a feature of the plasma torch. Susceptible quartz tubes or quartz domes for generating plasmas have to be avoided. There is a plasma torch aimed at, which is simple in its entire setup.
According to the invention the object is realized by the features of the Patent claim. As a matter of fact, it is initially irrelevant whether or not the coaxial guide is, in a cross-coupling, directed transversally to the hollow guide or, in an axial coupling, in parallel to the hollow guide, whether consequently their longitudinal axes preferably include a right angle or whether or not their longitudinal axes substantially coincide. The plasma torch (plasma generator) comprises a vacuum chamber and a magnetron, which within the vacuum chamber generates itself a field intensity sufficient for plasma formation. A recipient succeeding the coaxial guide is under a pressure of 100 Pa to 10 kPa, this pressure is suited for the formation of a plasma. A high efficiency is attained irrespective of the kind of coupling. The inventional plasma torch does without a cooling and without magnet coils due to its simple axial setup with an antenna as an electrode. The advantage in using a hollow wave guide instead of an a. c.-waveguide lies in the fact that the microwave output is not only coupled in the plasma in the vicinity of the nozzle, where there are the highest field intensities, but via the hollow space waves along the entire hollow guide axis. Such a design permits a quasi-electrodeless coupling-in that reduces the thermal stress of the nozzle. Advantageously, the hollow electrode is designed as a truncated cone and secured to a non-conductive intermediate member that is connected to the coaxial guide via a preferably disk-shaped mount. The nozzle is connected to a gas inlet through this intermediate member. The mounting disk is flanged to the coaxial guide and to the hollow guide. Advantageously, the hollow electrode is designed as a truncated cone, the shell of which is in opposition to the recipient. The hollow electrode is provided with an exchangeable nozzle that is inserted, preferably screwed into the inside space; the nozzle comprises four exit orifices for the operation gas, the exit orifices are arranged in the exit plane, regularly spaced from each other on a circle centered about the exit plane. In this way, an optimal directing of the microwave to the exit plane (nozzle tip) is achieved and a favorable energy input into the plasma flame is attained. A nozzle adapted for high temperatures preferably consists of a metal-ceramic alloy. An electrically non-conductive insulator thermally insulates the space of the plasma flame from the coupling site. An advantageous solution for the operation of the plasma torch is obtained in rendering the electrode axially and, if necessary, radially adjustable. In the case of cross-couplings, a brass member and a second intermediate member preferably connect the nozzle and the first intermediate member to a gas inlet. The brass member in any case ensures the electromagnetic coupling of the hollow conductor and coaxial guide. The hollow guide, preferably a rectangular hollow guide, of the cross-coupling is provided with two screws for tuning the electromagnetic wave to the coupling. In the case of the hollow guide, preferably a round hollow guide, of the axial coupling, the tuning is advantageously carried out in that its length is variable. To this end the hollow guide consists of, for example, two parts that can be telescope like slid one into the other, also during operation. One of the tubes can be provided with longitudinal slots and in-between remaining resilient lugs. A microwave seal is advantageously provided in an annular groove located between the tubes in an overlapping range. At the transition from the coaxial guide to the recipient a vacuum passageway for the electrode and the operation gas is provided; in this way an efficient coupling of the electromagnetic wave is obtained.